Internal combustion engines operated in individual-cylinder fashion often have a so-called fuel quantity compensation control system (FQCC) or smooth-running control system (SRC) which is described, for example, in published German patent document DE 199 45 618. For this, a controller is associated with each cylinder of the internal combustion engine. The background for this action is that in the event of any quantity errors during fuel metering, an inherently undesirable torque nonuniformity occurs. If an increased quantity of fuel is metered to a cylinder as a result of tolerances, the FQCC causes a negative fuel quantity to be added to the operator-commanded quantity for that cylinder. Conversely, a positive fuel quantity is added when too small a fuel quantity is metered to a cylinder.
During operation of a diesel engine, by means of the two control systems (FQCC and SRC) any injection quantity errors that result in torque differences, and thus in nonuniform engine rotation speeds, can be corrected, and engine smoothness at low rotation speeds—which is known to be critical in diesel engines—can be substantially improved. In addition, the equalization of the cylinders caused by the correction allows emissions improvements to be achieved not only in the lower rotation-speed range relevant to smoothness, but also at higher engine speeds.
A fuel quantity equalization by means of an FQCC or SRC is not efficient in all operating ranges of a diesel engine, however, since depending on the type of engine, additional effects occur, such as torsional vibrations at the crankshaft, which moreover are highly dependent on rotation speed. Methods for individual-cylinder operation of an internal combustion engine have therefore also been proposed in which a direct evaluation is made of a signal (for example of the cylinder pressure) that is directly correlated with combustion. These methods allows cylinder equalization even at higher rotation speeds. In these methods, operating parameters of the combustion process, for example the mean pressure indicated in the individual cylinders or the torque corresponding thereto, or the center point of the pressure difference between the individual cylinders, are calculated from the cylinder pressure profile. By regulating the indicated mean pressure, it is thus possible to achieve a more precise individual-cylinder regulation of the setpoint torques, and thus a better equalization of the cylinders in all operating ranges.
In cylinder-pressure-based engine control systems, the cylinder pressures occurring during operation of the engine are measured by means of pressure sensors over time, or as a function of crank angle, and made available to an engine control system. A pressure sensor of this kind is described, for example, in published German patent document DE 197 49 814. Also known are methods with which parameters can be ascertained during engine operation from a high-resolution pressure signal and are in turn used for individual-cylinder optimization of the engine process in terms of the target variables of fuel economy, emissions, and smoothness. The parameters used are, for example, the pressure or pressure difference integrals, the indicated work, or the indicated torque.
Also disclosed, in the German patent application 101 59 017, is a similar method in which open-and/or closed-loop control of engine operating parameters is accomplished as a function of a variable derived from the actual pressure signal. This derived variable characterizes, for example, the change in the pressure signal or the combustion profile. This enables a very accurate correction of preinjection during engine operation.
The aforesaid individual-cylinder control operation is usually accomplished by means of positioners, disposed in or on the cylinders, that are triggered by way of individual-cylinder control variables. These control variables are, for example, the triggering durations and/or triggering onsets of an injection.
The cylinder-pressure-based methods have the disadvantage that the measured values supplied by the pressure sensors are often erroneous as a result of tolerances related to manufacture and/or operation. If these measurement errors are not corrected, they distort the calculated cylinder pressure values and, because of the aforesaid control operation, result in mistuning among the individual cylinders.
The aforesaid measurement errors are expressed in differing sensor parameters that are incorporated into sensor characteristic curves both as an erroneous offset and as an erroneous gain factor. Suggested solutions for calibrating or correcting the offset already exist, in which the pressure rise at the beginning of a compression phase of the internal combustion engine is evaluated. No methods so far exist, however, for calibrating or correcting any erroneous gain factors. The individual gain factors moreover play a higher-order role in the overall operation of an internal combustion engine, since they are directly involved in the calculation of many other cylinder pressure features as well.
It is therefore an object of the present invention to provide a method for the calibration of sensors, disposed in at least two cylinders of an internal combustion engine operating in individual-cylinder fashion, for sensing a variable characterizing the combustion process in the respective cylinder, that makes possible adjustment of the at least two sensors with greater precision.